Which bodyforce is best for Bullets, Flying balls, etc?

Your best bet is probably going to be BodyVelocity. BodyVelocity sends the object at a certain speed in a certain direction. These are both determined in the same value "Velocity", which is sort of like the slope in three dimensions. This is the best choice, unless you want them to be homing missiles or include acceleration.

Here's an example of how to create your missile:

--local script in StarterGui

local player = game.Players.LocalPlayer --define the player
local character = player.Character --define the character
local mouse = player:GetMouse() --define the player's mouse

local missile = Instance.new("Part") --define our missile
missile.Shape = 0
missile.BrickColor = BrickColor.new("Dark indigo")
missile.Transparency = .5
missile.Size = Vector3.new(1,1,1)
missile.TopSurface = "Smooth"
missile.BottomSurface = "Smooth"
local f = Instance.new("Fire",missile)--add fire trail
f.Color = Color3.new(1,0,1)
f.SecondaryColor = Color3.new(1,0,1)

mouse.Button1Down:connect(function()--fire function when player clicks mouse
    local start = character.Head.Position + Vector3.new(0,3,0) --where we want the missile to start at, 3 studs above the player's head
    local target = mouse.Hit.p --the location the mouse is pointing at
    local clone = missile:Clone() --clone the missile
    local bv = Instance.new("BodyVelocity",clone) --create our body velocity
    bv.Velocity = (target - start).unit*50 --change the velocity value
    --(target - start).unit is the direction we want to fire the missile at, and *50 is the speed we want it to travel at.

    clone.Touched:connect(function(hit)--fire function when missile is touched
        if hit:IsDescendantOf(character) then--fire given code if touched part is a descendant of the character who fired the missile
            return--terminate code
        end
        local b = Instance.new("Explosion")--create explosion
        b.Position = clone.Position--position explosion at missile
        b.Parent = workspace--put explosion in workspace
        clone:Destroy()--destroy missile
    end)

    clone.CFrame = CFrame.new(start,target)*CFrame.Angles(math.rad(90),0,0) --Positions the missile above the player. We don't have to rotate it, but it prevents the fire from looking odd as the missile travels.
    clone.Parent = workspace --put missile in the workspace
    wait(10)--yield code for 10 seconds
    if clone then--if missile still exists then fire given code
        local b = Instance.new("Explosion")--create explosion
        b.Position = clone.Position--position explosion at missile
        b.Parent = workspace--put explosion in workspace
        clone:Destroy()--destroy missile
    end
end)